It is highly desirable to brake a rolling slatted door just before full door closure to reduce its downward velocity and therefore preclude the possibility of buckling the door upon full closure. Such braking eliminates the need for increased structural strength to withstand impact loading which would otherwise occur.
In the present art , this braking action is usually accomplished by changing the state of the contacts of an electrical limit switch when the door reaches a predetermined height near its fully closed position. The activation of the limit switch then activates a brake on the door's roller, slowing the door while still maintaining tension during its final closure. If applicable, power to the door's motor is also cut simultaneously by activation of the limit switch.
The design works well during powered operation of the door and during emergency operation of the door as long as electrical power is available. However, building fires are often preceded by electrical power failure, therefore failsafe operation of fire doors during an unpowered condition still requires that they be designed for buckling loads on impact. Battery backup for the electrical limit switches and associated system components has been suggested to overcome this problem, however, such battery backup is frowned upon because of the possibility of several undetermined existing problems.
In a conventional art by McKeon Rolling Steel Door Co., Inc., 95 29th Street, Brooklyn, N.Y. 11232, as shown in the Catalog thereof, in order to counterbalance the weight of the curtains at every point of travel, doors are counterbalanced by means of adjustable steel helical torsion springs attached to a shaft enclosed in a pipe with required mounting blocks or rings for attachment of curtains.
With the advent of the failsafe fire door release mechanism having automatic reset as described by McKeon in U.S. patent application Ser. No. 5,245,879, granted Sep. 21, 1993, a need arose to have an associated failsafe automatically resettable limit switch which would activate the unit's brake to brake the door when the door reached a predetermined point during its closure, yet be fully and automatically resettable during powered operation by simply pressing the "UP" button on the door control panel, thus allowing full normal operation of the door without any further intervention by the person operating the door.